1. Field of the Invention
Aspects of the present invention relate to detention basin outlet control structures that are designed to throttle stormwater flow and maintain the rate at which water is discharged from the detention basin below Qcritical, a flow rate at which erosion and down cutting of the receiving stream would begin.
2. Description of the Related Art
Conventional stormwater detention basins are ubiquitous in the developed portions of the U.S., particularly those areas developed since the 1980s. They are abundant stormwater assets that are not being used to their fullest potential, as many traditional flood control detention basins were overly designed for flood control alone and do not include any measures to mitigate erosive flows causing downstream channel instability from hydromodification. This retrofit technology will utilize the excess capacity to optimize detention basins and lessen erosive flows without adversely impacting the flood control capacity of the facility. This technology can be utilized by the Environmental Protection Agency, other state and local government agencies, and the public throughout all developed areas of the U.S.
Urbanization and increased imperviousness alter the hydrology of a watershed, leading to increased runoff volumes, higher and/or longer lasting peak flows, and more frequent runoff events. These hydromodifications can result in accelerated stream bank erosion, stream bed down cutting, and stream instability, and these physical alterations to the stream channel negatively impact water quality (i.e., increased suspended solids), biological communities (through habitat disruption and/or loss) and can endanger infrastructure (i.e., utility lines, roads, bridges, etc.) located adjacent to streams, necessitating costly repairs. Analysis by numerous researchers demonstrates that conventional detention basins designed for peak-flow control have little to no attenuating effect on 97-99% of precipitation volume in a typical year (Emerson et al., 2003; Hawley, 2012) and can cause substantial increases in durations of geomorphically-effective flows that result in corresponding channel instability and enlargement (MacRae, 1997). Because a large portion of the erosive work done on many streams occurs at flows less than the 2-year peak flow (typically the smallest regulated storm in conventional detention basins), retrofit devices that reduce the release rate of these more frequent, erosive flows could be extremely cost-effective at minimizing downstream channel erosion. By installing a relatively inexpensive retrofit technology on an otherwise large, underutilized, and relatively expensive asset, conventional detention basins could be converted into extremely effective assets in improving the nation's urban and suburban waters.
Communities across the nation need to revisit their stormwater regulations and post construction requirements to meet channel protection requirements. A device like this detention pond retrofit technology could have significant implications to government agencies and the public when a government agency begins to mandate management of existing impervious areas, which is already being done, for example, in Maryland. Similar to the presumptive approaches for stormwater management, presumptive approaches for managing impervious areas could very likely be both expensive and ineffective at solving the problem of channel instability. In contrast, calibrating regionally-appropriate solutions and installing green retrofits for detention basin outlet control structures could demonstrate that a regional approach to master planning and retrofitting can be much more cost-effective. Such a device could provide a way for governments and stormwater utilities to spend less money to solve ecologic and water quality problems, but the money would also be spent in ways that protect public infrastructure and results in even more long-term savings to the general public.